Maintenance of cellular homeostasis requires synthesis of new proteins and organelles and efficient removal of worn out cell components. Degradation processes are carried via two mechanisms, the ubiquitin-proteasome pathway and the autophagy-lysosome pathway. Defects in both of these catabolic pathways have been linked to pathogenesis of a variety of diseases. Autophagic function declines with age leading to accumulation of cellular debris, injury and cell senescence or death. This observation leads to the central hypothesis of this application, that autophagic dysfunction contributes to the late onset and/or acute exacerbations typical of interstitial lung disease (ILD). We postulate that autophagic dysfunction in alveolar epithelial cells leads to chronic epithelial cell injury that, in turn, promotes aberrant fibrotic repair. Surfactant protein C (SP-C) represents a highly tractable model to study the role of autophagy in ILD: SP-C expression in the lung is limited to the type II epithelial cell and mutations in the gene encoding SP-C (SFTPC) are associated with sporadic and heritable ILD. In this application, we present preliminary findings that specific SFTPC mutations are degraded exclusively by autophagy and additional new data implicating autophagy in the pathogenesis of ILD. Three specific aims are proposed to study SFTPC mutations in transfected cells, novel transgenic mouse models and lung tissue from human ILD patients, with the goal of identifying the molecular pathway(s) that targets mutant SP-C to autophagy and the role of this pathway in the pathogenesis of ILD. Specific Aim 1 will test the hypothesis that aggregation-prone, mutant SP-C proteins are selectively degraded by autophagy and that inhibition of this pathway leads to accumulation of cytotoxic SP-C. Specific Aim 2 will test the hypothesis that a novel quality control pathway in the endoplasmic reticulum mediates rapid identification and delivery of SP-C to the autophagic pathway. Specific Aim 3 will test the hypothesis that autophagic dysfunction in novel transgenic mouse models leads to epithelial cell injury that, in turn, promotes the onset and/or exacerbation of ILD. These studies will provide new diagnostic tools, new therapeutic targets and appropriate animal models to facilitate identification of pathogenetic pathways and development of novel treatment paradigms for ILD.